1. Field of the Invention
This invention relates to solenoid rotary actuators and, in particular, to a rotary actuator having an actuating coil and a permanent magnet rotor capable of bidirectional torque.
2. Discussion of Prior Art
U.S. Pat. No. 3,435,394 issued to Egger on Mar. 25, 1969 discloses a number of embodiments which can be described as electromagnetic control devices. Devices similar to these are now being marketed under the name brushless torque actuators by Lucas Ledex Inc. (the assignee of the present invention). These actuators generally comprise a single phase DC rotary solenoid incorporating a rotary element which is electrically operable in only one direction regardless of coil polarity. Upon energization of the electromagnet, the rotationally moveable pole piece is attracted to rotate to a position which minimizes the air gap over which flux has to flow in the electromagnetic circuit of the device. This causes a resultant rotation of the shaft in a predetermined direction.
Egger discloses a number of different rotor and stator configurations which provide a variety of torque versus angular rotation curves. The amount of rotation is based upon the torque generated and a spring which resists rotation. By changing the energization level of the coil, the device can be made to rotate a desired angular amount. Unfortunately, because Egger operates only upon the principle of increasing permeability (decreasing the air gap), it operates exactly the same regardless of the polarity of current flowing through the coil.
Another rotational actuator which has recently become available is that provided by Moving Magnet Technologies (MMT) of Besancon, France and is illustrated in FIGS. 1 and 2. The MMT actuator is a single phase DC coil actuator having a limited total rotational angle of approximately 110.degree. and is bi-directional. The MMT is shown generally at 10 in FIG. 1 and in an exploded view in FIG. 2.
Separate coils 12 and 14 are wound around separate stators 16 and 18. The coils are wound and/or energized so as to polarize the stators in opposite directions. The stators and the end plate 20 are of ferrous material which is a good conductor of electromagnetic flux. The MMT actuator case 22 is a non-magnetic sleeve into which the coils may be bonded. An output shaft 24 has a pair of permanent magnets 26 and 28 bonded thereto. The shaft is mounted for rotation in base 20 and in sleeve 22 with appropriate bearings (not shown). The direction of polarization of both magnets 26 and 28 is parallel to the output shaft 24 and its axis of rotation. However, the polarization of magnet 26 is directly opposite the polarization of magnet 28. Also, connected to the output shaft and in contact with the magnets 26 and 28 is a ferrous flux carrier 30.
By review of FIG. 2, it can be seen that when there is no energization of the electromagnetic coils, there is essentially no net torque applied to the output shaft since permanent magnets 26 and 28 are merely attracted in the axial direction towards the stators 16 and 18. However, when the coils are energized so as to generate opposite polarity magnetic flux fields (as shown in FIG. 2), and when the junction between magnets 26 and 28 is directed generally towards the midpoint of stators 16 and 18, a net rotational force is generated on the output shaft.
The lower surface of the permanent magnet 26 has a "north" polarity and the upper surface of stator 16 has a "south" polarity and thus magnet 26 is attracted towards pole piece 16. Since the output shaft is constrained by bearings against axial movement, the shaft attempts to rotate so as to bring magnet 26 in line with stator 16. Also, a portion of magnet 28 also overlaps stator 16 but because they are of like polarity, magnet 28 will be repelled from stator 16. Thus, for stator 16, magnet 26 is attracted and magnet 28 is repelled and, because of the opposite polarity at stator 18, magnet 28 is attracted and magnet 26 is repelled. As a consequence, both magnets and both stators develop forces which result in a net rotation in the direction shown by arrows 32.
It can be seen that the magnetic flux path during energization of the MMT actuator, as illustrated in FIG. 2, is down through stator 16, across the ferromagnetic base, up through stator 18, across a working air gap, through the magnet 28, across the ferrous flux carrier 30, down through magnet 26, across a further working air gap and back to stator 16. Of course, should the current flow in electromagnetic coils 12 and 14 be reversed, the flux flow and the polarity at the top of stators 16 and 18 would be reversed and the rotational direction of the output shaft would also be reversed. Therefore, the MMT provides bi-directionality, dependent upon the energization direction of the electromagnet coils and also provides for an angular rotation of up to 90.degree. in each direction (although in actuality, the rotation is only approximately 55.degree.).
While the MMT actuator is an improvement over the Egger and other similar devices, because of the kidney shape of stators 16 and 18, to obtain the highest efficiency, coils 12 and 14 should be wound such that they conform to the kidney shape. Such a complex winding requires special handling and fixturing to form the coils properly. The coils can either be series or parallel wound. If the coils are series wound, the problems of coil winding are exacerbated although if they are parallel wound, two separate three wire connections will be necessary to connect the lead wires.
Also, there are disadvantages in the MMT actuator as a result of the requirement of flux carrier 30. This is necessary to close the magnetic flux circuit, as noted above, and must be mounted for rotation with the output shaft. Unfortunately, this ferrous material significantly increases the inertia of rotation and therefore the response of the actuator. The elimination of the ferrous flux carrier in the MMT would greatly reduce the torque available because the return path for the electromagnetic flux from the top of magnet 28 to the top of magnet 26 would be through air which has very poor flux conductivity. Therefore, the high inertia as a result of utilizing the ferrous flux carrier 30 is a consequence of the MMT actuator.
A further device which is of interest is the rotary actuator or magnetic spring disclosed in U.S. Pat. No. 5,038,063 issued to Graber et al on Aug. 6, 1991. Graber utilizes one shaft connected to a plurality of magnets where adjacent magnets have opposite polarities (just as in the MMT actuator). Sandwiching the plurality of magnets are magnetic pole pieces offset from each respecting opposing pole piece such that when energized, they tend to bias the position of the magnets with respect to the two disks of pole pieces. The strength of the magnets, the working air gaps involved, the stator pole offset angle and the external energization level serves to define the force tending to link the magnets with the pole pieces.
In a preferred embodiment, one shaft is connected to both sets of pole pieces and another shaft is connected to the magnets and the degree of coupling between the two shafts can be controlled by the energization level of the magnetic spring. It is noted that in the Graber device when operating as an actuator (or a magnetic spring for that matter), the poles as shown in FIGS. 4a through 4c are always displaced from each other and the junction between opposite polarity magnets in the rotor disk is never in line with the mid point of both upper and lower opposing stators. This offset (of one quarter pole pitch as discussed in column 3, line 63) is shown in each of Graber's Figures and is necessary in order to provide a magnetic "restoring (centering) force" as set forth in column 4, lines 16 through 23.
It is desirable to have a magnetically efficient brushless torque actuator which will operate in the fashion of an MMT actuator, i.e. is bi-directional depending upon the activating current but with relatively low inertia and therefore can respond quickly to changes in energizing current, amplitude or polarity.